The invention relates to a product comprising a tyrosinase inhibiting extract derived from a dicotyledonous plant species indigenous to Canada. The present invention also relates to compositions comprising the product and a suitable diluent or carrier for the treatment of skin or of fruit, vegetables or beverages. The present invention further relates to a method for identifying plant extracts that are capable of inhibiting the enzyme tyrosinase.
Tyrosinase (tyrosine oxidase, EC 1.10.3.1) is an enzyme or closely homologous group of enzymes of universal occurrence in microbes, plants and animals. Its primary metabolic function is to catalyze the oxidative degradation of the amino acid tyrosine. This degradation takes slightly different routes in animals, plants and microbes, but the rate-controlling first stepsxe2x80x94those catalyzed by tyrosinasexe2x80x94are the same in virtually all living species. In animals, including man, tyrosinase first transforms tyrosine into 3,4-dihydroxyphenylalanine (DOPA), thence to the corresponding quinone (DOPAquinone), and finally to 2-carboxy-2,3-dihydroindole-5,6-quinone (DOPAchrome) which is further converted by other enzymes to still more highly oxidized materials which include the melanin substances responsible for skin pigmentation.
Skin pigmentation thus depends upon the action of tyrosinase. If this enzyme is not active, normal pigmentation does not occur, and skin loses or fails to acquire its normal tan-to-brown coloration. The effect is independent of racial or environmental factors. Pigmentation loss manifests itself in lentigo senile, so-called xe2x80x9cage spotsxe2x80x9d, a small-scale, patchy color loss often seen in skin of people over 50 years of age; typically the de-pigmented patches will be 1-3 mm across and can be very numerous. A more serious effect is leucodenna, a group of diseases in which large areas of skin lose their melanin and appear pink. An extreme effect is albinism, a condition in which tyrosinase enzyme is entirely dysfunctional and no pigmentation of skin (or eyes or hair) occurs.
If on the other hand tyrosinase activity is accelerated, as in sun-tanning or in some pathological conditions, the amount of melanin formed increases and skin color darkens. When the distribution of the new melanin is even and controlled, the result is a xe2x80x9cglorious bronzed bodyxe2x80x9d; but if the new melanin is patchy or produced uncontrollably, the result is pathological. Skin melanomas are sites of localized hyperactivity by tyrosinase; they are often associated with cancerous cell modification. The causal relationship of tyrosinase action to skin pigmentation was established many years ago (Mason, H. S. (1948) J. Biol. Chem. 172, 83-86; Balin, A. K. and Kligman, A. M. (1989) Aging and the Skin 372pp, Raven Press, N.Y.) and is now well-understood.
Materials which suppress the action of tyrosine oxidase are presumed to slow the build-up of skin spots over time, and this presumption is vindicated by scientific evidence. Thus, the correlation between tyrosinase inhibition and protection of skin against unwanted pigmentation is accepted by the medical profession and by the cosmetic industry.
In addition to its skin functions, tyrosinase is active in other types of living tissue in the turnover metabolism of tyrosine and in the production of pigmented materials in those tissues.
With respect to cosmetics, skin lightening and skin darkening have been practiced since earliest times, and today these are mainstays of some sectors of the world cosmetics industry. Skin lightening has traditionally been accomplished by rigorously excluding sunlight from skin, or by the use of chemical lightening agents. One such agent is synthetic hydroquinone (1,4-dihydroxybenzene). This substance is considered the cosmetic industry standard for tyrosinase inhibition potency.
Prior to the present invention, the possibility of finding effective tyrosinase inhibitors in dicotyledonous plant species indigenous to Canada had not been appreciated.
Accordingly, the present invention provides a product comprising an extract derived from a dicotyledonous plant species indigenous to Canada, wherein the extract inhibits tyrosinase. Preferably, the extract is derived from a plant species selected from Polygonaceae, Rosaceae and Onagraceae. The extract is preferably derived from one or more parts of the plant selected from leaves, twigs, flowers, flowering aerials, fruiting aerials, seeding aerials, roots and fruits.
Preferably, the extract is derived from the group consisting of:
flowering aerials of Artemisia campestris (plains wormwood);
flowering aerials of Aster ericoides (white prairie aster);
flowering aerials of Aster hesperius (willow aster);
leaves, twigs and flowers of Cornus stolonifera (red-osier dogwood);
leaves and twigs of Cotoneaster acutifolia (cotoneaster);
flowering aerials of Epilobium angustifolium (fireweed);
seeding aerials of Euphorbia esula (leafy spurge);
fruiting aerials of Fragaria americana (wild strawberry);
fruiting aerials of Fragaria glauca (Wild strawberry);
flowering aerials of Geranium bicknelli (Bicknell""s geranium);
flowering aerials of Geum aleppicum (yellow avens);
flowering aerials of Geum triflorum (3-flowered avens);
flowering aerials of Glycyrrhiza lepidota (wild licorice);
flowering aerials of Hedysarum americanum (American hedysarum); 
roots of Heuchera richardsonii (alumroot);
flowering aerials of Oenothera biennis (yellow evening-primrose); flowering aerials of Polygonum persicaria (lady""s-thumb);
flowering aerials of Potentilla fruticosa (shrubby cinquefoil);
flowering aerials of Potentilla norvegica (rough cinquefoil);
flowering aerials of Rosa acicularis (prairie rose);
fruiting aerials of Rosa arkansana (low rose);
fruiting aerials of Rumex maritimus (golden dock);
fruiting aerials and flowering aerials of Rumex occidentalis (western field dock);
fruiting aerials and fruits of Rumex pseudonatronatus (field dock); and
fruits of Rumex stenophyllus (narrow-leaved dock).
The present invention also provides a composition comprising the product as defined herein, together with a cosmetically or pharmaceutically acceptable, or edible, diluent or carrier. The composition is preferably for cosmetic treatment of skin or for inhibiting browning of edible products. The composition is more preferably for skin lightening, even more preferably to reduce melanin and/or melanogenesis. The at least one plant extract in the composition is preferably selected from the group consisting of:
fruiting aerials of Rumex maritimus (golden dock);
fruiting aerials of Rumex occidentalis (western field dock);
flowering aerials of Rumex occidentalis (western field dock);
fruiting aerials of Rumex pseudonatronatus (field dock);
fruits of Rumex pseudonatronatus (field dock); and
fruits of Rumex stenophyllus (narrow-leaved dock).
The present invention also provides a method for detecting tyrosinase inhibiting activity in an extract derived from a dicotyledonous plant species indigenous to Canada. This method comprises:
a) preparing a first solution comprising an amount of tyrosinase and a suitable substrate;
b) preparing a second solution comprising the same amount of tyrosinase and the suitable substrate and further comprising an amount of the extract;
c) measuring tyrosinase activities of the first and second solutions by suitable methods;
d) comparing the tyrosinase activities of the first and second solutions; and
e) detecting tyrosinase inhibiting activity, present when the tyrosinase activity of the second solution is less than the tyrosinase activity of the first solution.
It is preferable that the tyrosinase inhibiting activity is comparable to, or greater than, that of hydroquinone.